PHYSIOLOGICAL ANATOMY.
THE BONES.
All living bodies are made up of tissues. There is no part, no organ,
however soft and yielding, or hard and resisting, which has not this
peculiarity of structure. The _bones_ of animals, as well as their flesh
and fat, are composed of tissues, and all alike made up of cells. When
viewed under a microscope, each cell is seen to consist of three
distinct parts, a _nucleolus_, or dark spot, in the center of the cell,
around which lies a mass of granules, called the _nucleus;_ and this, in
turn, is surrounded with a delicate, transparent membrane, termed the
_envelope_. Each of the granules composing the nucleus assimilates
nourishment, thereby growing into an independent cell, which possesses a
triple organization similar to that of its parent, and in like manner
reproduces other cells.
[Illustration: Fig. 4.
Nucleated cell.
From Goeber.
1. Periphery of the
cell, or cell-wall.
2. Nucleus. 3. Nucleolus
in the center.]
A variety of tissues enters into the composition of an animal structure,
yet their differences are not always distinctly marked, since the
characteristics of some are not unlike those of others. We shall notice,
however, only the more important of the tissues.
The _Areolar_, or _Connective Tissue_, is a complete network of delicate
fibers, spread over the body, and serves to bind the various organs and
parts together. The fibrous and serous tissues are modifications of the
areolar.
The _Nervous Tissue_ is of two kinds: The gray, which is pulpy and
granulated, and the white fibrous tissue. The _Adipose Tissue_ is an
extremely thin membrane, composed of closed cells which contain fat. It
is found principally just beneath the skin, giving it a smooth, plump
appearance.
[Illustration: Fig. 5.
Arrangement of fibers in the
Areolar Tissue. Magnified 135 diameters.]
The _Cartilaginous Tissue_ consists of nucleated cells, and, with the
exception of bone, is the hardest part of the animal frame. The _Osseous
Tissue_, or bone, is more compact and solid than the cartilaginous, for
it contains a greater quantity of lime. The _Muscular Tissue_ is
composed of bundles of fibers, which are enclosed in a cellular
membrane.
[Illustration: Fig. 6.
Human Adipose Tissue.]
Various opinions have been entertained in regard to the formation, or
growth, of bone. Some anatomists have supposed that all bone is formed
in cartilage. But this is not true, for there is an _intra-membranous_,
as well as an _intra-cartilaginous_, formation of bone, as may be seen
in the development of the cranial bones, where the gradual calcification
takes place upon the inner layers of the fibrous coverings.
Intra-cartilaginous deposit is found in the vicinity of the
blood-vessels, within the cartilaginous canals; also, there are certain
points first observed in the shafts of long bones, called _centers of
ossification_. These points are no sooner formed than the cartilage
corpuscles arrange themselves in concentric zones, and, lying in contact
with one another, become very compact. As ossification proceeds, the
cup-shaped cavities are converted into closed interstices of bone, with
extremely thin lamellæ, or layers. These, however, soon increase in
density, and no blood-vessels can be observed within them.
[Illustration: Fig. 7.
Vertical section of cartilage near the surface of
ossification. _1_. Ordinary appearance of the temporary
cartilage. _1_'. Portion of the same more
highly magnified. _2_. The cells beginning to form
into concentric zones. _2_'. Portion more magnified.
_3_. The ossification is extending in the inter-cellular
spaces, and the rows of cells are seen
resting in the cavities so formed, the nuclei being
more separated than above. _3_'. Portion of the
same more highly magnified.]
[Illustration: Fig. 8.
Thigh-bone,
sawn open
lengthwise.]
[Illustration: Fig. 9.
Lower end of the thigh-bone
sawn across, showing its central
cavity.]
The bony plates form the boundaries of the _Haversian_, or nutritive
canals of the bones. In the _second stage of ossification_, the
cartilage corpuscles are converted into bone. Becoming flattened against
the osseous lamellæ already formed, they crowd upon one another so as to
entirely obliterate the lines that distinguish them; and, simultaneously
with these changes, a calcareous deposit takes place upon their
interior. Bones grow by additions to their ends and surfaces. In the
child, their extremities are separated from the body of the bone by
layer of cartilage, and the cancellated, or cellular structure, which
remains for a time in the interior, represents the early condition of
the ossifying substances.
The bones contain more earthy matter in their composition than any other
part of the human body, being firm, hard, and of a lime color. They
compose the skeleton or frame work, and, when united by natural
ligaments, form what is known as the _natural_ skeleton; when they are
wired together, they are called an _artificial_ skeleton. The number of
bones in the human body is variously estimated; for those regarded as
single by some anatomists are considered by others to consist of several
distinct pieces. There are two hundred distinct bones in the human
skeleton besides the teeth. These may be divided into those of the Head,
Trunk, Upper Extremities, and Lower Extremities.
[Illustration: Fig. 10.
The bones of the skull separated. _1_. Frontal,
only half is seen. _2_. Parietal. _3_. Occipital, only
half is seen. _4_. Temporal. _5_. Nasal. _6_. Malar.
_7_. Superior maxillary (upper jaw). _8_. Lachrymal.
_9_. Inferior maxillary (lower jaw). Between
_4_ and _6_ a part of the sphenoid or wedge-shaped
bone, is seen. Another bone assisting to form
the skull, but not here seen, is called the _ethmoid_
(sieve-like, from being full of holes), and is situated
between the sockets of the eyes, forming the
roof of the nose.]
THE BONES OF THE HEAD are classed as follows: eight belonging to the
Cranium, and fourteen to the Face. The bones of the Cranium are the
_occipital_, two _parietal_, two _temporal, frontal, sphenoid_, and
_ethmoid_. Those composing the face are, the two _nasal_, two _superior
maxillary,_ two _lachrymal_, two _malar_ two _palate_, two _inferior
turbinated, vomer_, and _inferior maxillary_. The cranial bones are
composed of two dense plates, between which there is, in most places a
cancellated or cellular tissue. The external plate is fibrous, the
internal, compact and vitreous. The skull is nearly oval in form, convex
externally, the bone being much thicker at the base than elsewhere, and
it is, in every respect admirably adapted to resist any injury to which
it may be exposed, thus affording ample protection to the brain
substance which it envelops. The internal surface of the cranium
presents eminences and depressions for lodging the convolutions of the
brain, and numerous furrows for the ramifications of the blood-vessels.
The bones of the cranium are united to one another by ragged edges
called _sutures_, which are quite distinct in the child but which in old
age are nearly effaced. Some authorities suppose that by this
arrangement the cranium is less liable to be fractured by blows; others
think that the sutures allow the growth of these bones, which takes
place by a gradual osseous enlargement at the margins. The bones of the
_Face_ are joined at the lower part and in front of the cranium, and
serve for the attachment of powerful muscles which assist in the process
of mastication. Although the soft parts of the face cover the bony
structure, yet they do not conceal its principal features, or materially
change its proportions. The form of the head and face presents some
remarkable dissimilarities in different races.
[Illustration: Fig. 11.
_1_. The first bone of the sternum (breast-bone).
_2_. The second bone of the sternum.
_3_. The cartilage of the sternum. _4_. The
first dorsal vertebra (a bone of the spinal
column). _5_. The last dorsal vertebra. _6_.
The first rib. _7_. Its head. _8_. Its neck. _9_.
Its tubercle. _10_. The seventh or last true
rib. _11_. The cartilage of the third rib. _12._
The floating ribs.]
[Illustration: Fig. 12.
A vertebra of the neck. _1_. The
body of the vertebra. _2_. The spinal
canal. _4_. The spinous process
cleft at its extremity. _5_. The
transverse process. _7_. The interior
articular process. _8_. The
superior articular process.]
THE TRUNK has fifty-four bones, which are as follows: The _Os Hyoides_,
the _Sternum_, twenty-four Ribs, twenty-four _vertebræ_ or bones of the
Spinal Column, the _Sacrum_, the _Coccyx_, and two _Ossa Innominata_.
The _Os Hyoides_, situated at the base of the tongue, is the most
isolated bone of the skeleton, and serves for the attachment of muscles.
The _Sternum_, or breast-bone, in a child is composed of six pieces, in
the adult of three, which in old age are consolidated into one bone. The
_Ribs_ are thin, curved bones, being convex externally. There are twelve
on each side, and all are attached to the spinal column. The seven upper
ribs, which are united in front of the sternum, are termed _true_ ribs;
the next three, which are not attached to the sternum, but to one
another are called _false_ ribs; and the last two, which are joined only
to the vertebræ, are designated as _floating_ ribs. The first rib is the
shortest, and they increase in length as far as the eighth, after which
this order is reversed.
[Illustration: Fig. 13.
_1_. The cartilaginous substance
which connects the bodies of
the vertebræ. _2_. The body of the
vertebra. _3_. The spinous process.
_4,4_. The transverse processes.
_5,5_. The articular processes.
_6,6_. A portion of the bony bridge
which assists in forming the spinal
canal (7).]
[Illustration: Fig. 14.
Backbone, spinal
column, or vertebral
column. All
animals possessing
such a row of bones
are called _vertebrates_.
Above _b_ are
the cervical (neck)
vertebræ; _b_ to _c_,
dorsal (back) or
chest vertebræ; _c_
to _d_, lumbar (loins)
vertebræ; _d_ to _e_, sacrum;
_e_ to _f_, coccyx.]
The _Spinal Column_ or backbone, when viewed from the front presents a
perpendicular appearance, but a side view shows four distinct curves.
The bones composing it are called _vertebræ_. The body part of a
vertebra is light and spongy in texture, having seven projections called
_processes_, four of which are the _articular_ processes, which furnish
surfaces to join the different vertebræ of the spinal column. Two are
called _transverse_, and the remaining one is termed the _spinous_. The
transverse and spinous processes serve for the attachment of the muscles
belonging to the back. All these processes are more compact than the
body of the vertebra, and, when naturally connected, are so arranged as
to form a tube which contains the _medulla spinalis_, or spinal cord.
Between the vertebræ is a highly-elastic, cartilaginous and cushion-like
substance, which freely admits of motion, and allows the spine to bend
as occasion requires. The natural curvatures of the spinal column
diminish the shock produced by falling, running or leaping, which would
otherwise be more directly transmitted to the brain. The ribs at the
sides, the sternum in front, and the twelve dorsal bones of the spinal
column behind, bound the thoracic cavity, which contains the lungs,
heart, and large blood-vessels.
[Illustration: Fig. 15.
A representation of the pelvic bones. _e_. The
lumbo-sacral joint. 2. The sacrum. _3_. Coccyx. _1,1_.
The innominata. _4,4_. Acetabula.]
The _Pelvis_ is an open bony structure, consisting of the Os Innominata,
one on either side, and the Sacrum and Coccyx behind. The _Sacrum_,
during childhood, consists of five bones, which in later years unite to
form one bone. It is light and spongy in texture, and the upper surface
articulates with the lowest vertebra, while it is united at its inferior
margin to the coccyx. The _Coccyx_ is the terminal bone of the spinal
column. In infancy it is cartilaginous and composed of several pieces,
but in the adult these unite and form one bone. The _Innominata_, or
nameless bones, during youth, consist of three separate pieces on each
side; but as age advances they coalesce and form one bone. A deep
socket, called the _acetabulum_, is found near their junction, which
serves for the reception of the head of the thigh-bone.
[Illustration: Fig. 16.
1. Portions of the backbone. 2. Cranial
bones. _4_. Breast-bone. _5_. Ribs. _7_. Collar-bone.
_8_. Arm-bone (humerus). _9_. Shoulder-joint.
_10, 11_. Bones of the fore-arm (ulna and
radius). _12_. Elbow-joint. _13_. Wrist-joint. _14_.
Bones of the hand. _15, 16_. Pelvic bones. _17_.
Hip-joint. _18_. Femur. _19, 20_. Bones of the
knee-joint. _21, 22_. Fibula and tibia. _23_. Ankle
bone. _24_. Bones of the foot.]
THE BONES OF THE UPPER EXTREMITIES are sixty-four in number, and are
classified as follows: The Scapula, Clavicle, Humerus, Ulna, Radius,
Carpus, Metacarpus, and Phalanges. The _Scapula_, or shoulder-blade, is
an irregular, thin, triangular bone, situated at the posterior part of
the shoulder, and attached to the upper and back part of the chest. The
_Clavicle_, or collar-bone, is located at the upper part of the chest,
between the sternum and scapula, and connects with both. Its form
resembles that of the italic letter _f_, and it prevents the arms from
sliding forward. The _Humerus_, the first bone of the arm, is long,
cylindrical, and situated between the scapula and fore-arm. The _Ulna_
is nearly parallel with the radius, and situated on the inner side of
the fore-arm. It is the longer and larger of the two bones, and in its
articulation with the humerus, forms a perfect hinge-joint. The
_Radius_, so called from its resemblance to a spoke, is on the outer
side of the fore-arm, and articulates with the bones of the wrist,
forming a joint. The ulna and radius also articulate with each other at
their extremities. The _Carpus_, or wrist, consists of eight bones,
arranged in two rows. The _Metacarpus_, or palm of the hand, is composed
of five bones situated between the carpus and fingers. The _Phalanges_,
fourteen in number, are the bones of the fingers and thumb, the fingers
each having three and the thumb two.
THE BONES OF THE LOWER EXTREMITIES, sixty in number, are classed as
follows: The Femur, Patella, Tibia, Fibula, Tarsus, Metatarsus, and
Phalanges. The _Femur_, or thigh-bone, is the longest bone in the body.
It has a large round head, which is received into the acetabulum, thus
affording a good illustration of a ball and socket joint. The _Patella,_
or knee-pan, is the most complicated articulation of the body. It is of
a round form, connects with the tibia by means of a strong ligament, and
serves to protect the front of the joint, and to increase the leverage
of the muscles attached to it, by causing them to act at a greater
angle. The _Tibia_, or shin bone, is enlarged at each extremity and
articulates with the femur above and the astragalus, the upper bone of
the tarsus, below. The _Fibula_, the small bone of the leg, is situated
on the outer side of the tibia, and is firmly bound to it at each
extremity. The _Tarsus_, or instep, is composed of seven bones, and
corresponds to the carpus of the upper extremities. The _Metatarsus_,
the middle of the foot, bears a dose resemblance to the metacarpus, and
consists of five bones situated between the tarsus and the phalanges.
The tarsal and the metatarsal bones are so united as to give an arched
appearance to the foot, thus imparting elasticity. The _Phalanges_, the
toes, consist of fourteen bones, arranged in a manner similar to that of
the fingers.
We are not less interested in tracing the formation of bone through its
several stages, than in considering other parts of the human system. The
formation of the Haversian canals for the passage of blood-vessels to
nourish the bones, the earlier construction of bony tissue by a
metamorphosis of cartilaginous substance, and also the commencement of
ossification at distinct points, called _centers of ossification_, are
all important subjects, requiring the student's careful attention. The
bones are protected by an external membranous envelope, which, from its
situation is called the _periosteum_. The bones are divided into four
classes, _long, short, flat_ and _irregular_, being thus adapted to
subserve a variety of purposes.
The Long Bones are found in the limbs, where they act as levers to
sustain the body and aid in locomotion. Each_long_ bone is composed of a
cylinder, known as the _shaft_, and two _extremities_. The shaft is
hollow, its wails being _thickest_ in THE middle and growing thinner
toward the extremities. The _extremities_ are usually considerably
enlarged, for convenience of connection with other bones, and to afford
a broad surface for the attachment of muscles. The clavical, humerus,
radius, ulna, femur, tibia, fibula, the bones of the metacarpus,
metatarsus and the phalanges, are classed as long bones.
Where the principal object to be attained is strength, and the motion of
the skeleton is limited, the individual bones are short and compressed,
as the bones of the carpus and tarsus. The structure of these bones is
spongy, except at the surface, where there is a thin crust of compact
matter.
[Illustration: Fig. 17.
Anatomy of a joint, _1, 1_.
Bones of a joint. _2, 2_. Cartilage.
_3, 3, 3, 3_. Synovial
membrane.]
[Illustration: Fig. 18.
Anatomy of knee joint.
_1._ Lower end of thigh-bone.
_3._ Knee-pan. _2, 4_ Ligaments
of the knee-pan. _5_. Upper
end of the tibia, or shin-bone.
_6, 12_. Cartilages.]
When protection is required for the organs of the body, or a broad flat
surface for the attachment of the muscles, the bones are expanded into
plates, as in the cranium and shoulder-blades.
The _irregular_ or _mixed_ bones are those which, from their peculiar
shape, cannot be classed among any of the foregoing divisions. Their
structure is similar to the others, consisting of cancellar tissue,
surrounded by a crust of compact matter.
The vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two
maxillary, palate, inferior turbinated, and hyoid are known as irregular
bones.
The formation of the joints requires not only bones, but also
cartilages, ligaments, and the synovial membrane, to complete the
articulation. _Cartilage_ is a smooth, elastic substance, softer than
bone, and invested with a thin membrane, called _perichondrium_. When
cartilage is placed upon convex surfaces, the reverse is true. The
_Ligaments_ are white, inelastic, tendinous substances, softer than
cartilage, but harder than membrane. Their function is to bind together
the bones. The _Synovial Membrane_ covers the cartilages, and is then
reflected upon the ligaments, thus forming a thin, closed sac, called
the _synovial capsule._
All the synovial membranes secrete a lubricating fluid, termed
_synovia_, which enables the surfaces of the bones and ligaments to move
freely upon one another. When this fluid is secreted in excessive
quantities, it produces a disease known as "dropsy of the joints." There
are numerous smaller sacs besides the synovial, called _bursæ mucosæ_,
which in structure are analogous to them, and secrete a similar fluid.
Some joints permit motion in every direction, as the shoulders, some in
two directions only, as the elbows, while others do not admit of any
movement. The bones, ligaments, cartilages, and synovial membrane, are
supplied with nerves, arteries, and veins.
When an animal is provided with an internal bony structure, it indicates
a high rank in the scale of organization. An elaborate texture of bone
is found in no class below the vertebrates. Even in the lower order of
this sub-kingdom, which is the highest of animals, bone does not exist,
as is the case in some tribes of fishes, such as sharks, etc., and in
all classes below that of the cartilaginous fishes, the inflexible
substance which sustains the soft parts is either shell or some
modification of bone, and is usually found on the outside of the body.
True bone, on the contrary, is found in the interior, and, therefore, in
higher animals, the skeleton is always internal, while the soft parts
are placed external to the bony frame. While many animals of the lowest
species, being composed of soft gelatinous matter, are buoyant in water,
the highest type of animals requires not only a bony skeleton, but also
a flexible, muscular system, for locomotion in the water or upon the
land. Each species of the animal kingdom is thus organically adapted to
its condition and sphere of life.
* * * * *